1. Field of the Invention
The present invention relates to an image display method, for computer graphics (CG), for displaying an object constituted by polygons and an image display apparatus therefor.
2. Related Arts
According to the techniques for computer graphics (CG), an object in virtual three-dimensional space is constituted by at least one polygon. The object data consists of a set of polygon data, including vertex data for the polygon, and the polygon data are stored on a storage medium, such as a CD-ROM. When an image display apparatus reads the polygon data from the storage medium, the coordinates of the polygon data are transformed, which is required for the rendering process (a geometry process). At this time, the polygon data are sequentially read from the storage medium an transmitted to a geometry processing unit, which is provided in the image display apparatus and which performs the geometry process. Then, the geometry processing unit performs a predetermined coordinate transformation.
Conventionally, two formats are employed for the polygon data stored in the storage medium. FIG. 6 is a diagram employed for explaining the conventional formats used for polygon data. An explanation will now be given of the format employed for data for four triangular polygons, A, B, C and D, which constitute, as an object, a tetrahedron P, and for which vertexes are used in common.
In FIG. 6, the tetrahedron P has four vertexes, V0, V1, V2 and V3. The vertexes V0, V1, V3 and V3 are presented as vertex coordinate data by employing three-dimensional coordinates, which are defined as
V0=(x0, y0, z0),
V1=(x1, y1, z1),
V2=(x2, y2, z2),
V3=(x3, y3, z3).
With a conventional first format (hereinafter referred to as an index format), the vertex data for the individual polygons consist of the vertex coordinate data for the individual vertexes and the following vertex string data. That is,
polygon A=(V0, V1, V2),
polygon B=(V0, V3, V1),
polygon C=(V0, V2, V3),
polygon D=(V1, V3, V2).
Therefore, the triangular polygons A, B, C and D are defined by the vertex string data for three vertexes.
The vertexes in the vertex string data are transmitted to the geometry processing unit, beginning with the vertex string data for polygon A, in the order of V0, V1, V2, V0, V3, V1, V0, V2, . . . . Upon receiving each vertex, the geometry processing unit refers to the vertex coordinate data and reads from that data the coordinates of a vertex. The geometry processing unit, in accordance with the vertex coordinate data, then performs a predetermined coordinate transformation.
When the polygons, as in the tetrahedron P, are located adjacent to each other, and their vertexes are commonly employed, in the index format the vertex coordinate data are not provided for each of the vertexes which are received. Thus, the amount of required polygon data can be reduced.
However, since each time a vertex is received the corresponding vertex coordinate data must be referred to, the speed attained by geometry processing is reduced.
With a conventional second format (hereinafter referred to as a beta format), the vertex data for the individual polygons are provided directly as vertex coordinate data for the vertexes included in the vertex string data. That is, the polygon data are defined as
polygon A=(x0, y0, z0), (x1, y1, z1), (x2, y2, z2),
polygon B=(x0, y0, z0), (x3, y3, z3), (x1, y1, z1),
polygon C=(x0, y0, z0), (x2, y2, z2), (x3, y3, z3),
polygon D=(x1, y1, z1), (x3, y3, z3), (x2, y2, z2).
The geometry processing unit performs a predetermined coordinate transformation for each of vertex coordinates which are input, beginning with the vertex coordinates of the polygon A, in the order x0, y0, z0, x1, y1, z1, x2, y2, z2, z0, y0, z0, x3, y3, z3, x1, y1, z1, . . . .
In the beta format, the vertex data for the polygon data are provided as a set of vertex coordinate data. Therefore, since the geometry processing unit need not refer to the vertex coordinate data as in the index format, the vertex coordinate data, which are sequentially received, can be processed at high speed.
However, when as is described above the vertexes of adjacent polygons are employed in common, the coordinate data for the overlapping vertexes are increased, so that an enormous amount of polygon data is required.
As is described above, according to the index format, the processing speed can not be increased because the vertex coordinate data must be referred to, even though the amount of polygon data can reduced. According to the beta format, an enormous amount of polygon data is required, even though the processing speed can be increased.
It is, therefore, one objective of the present invention to provide an image display method using a new format according to which a high processing speed can be maintained and the amount of data can be reduced, and an image display apparatus therefor.
To achieve the objective, according to the present invention, it is provided an image display method for displaying an object constituted by a plurality of polygons comprising the steps of:
providing coordinate data for each vertex included in a plurarity of polygons; and
expressing data of a vertex in common to a plurarity of polygons by the coordinate data of the common vertex and position data for designating a position of the coordinate data of the common vertex.
Coordinate data disignated by the position data, or data which is coordinate-transformed from coordinate data disignated by the position data is used as the coordinate data of the common vertex corresponding to the position data.
Preferably, the coordinate data of the common vertex is read out earlier than the position data is read out.
Further, to achieve the above objective, according to the present invention, it is provided a first image display apparatus for displaying an object constituted by a plurality of polygons comprising:
a memory for storing data of vertex in common to a plurarity of polygons as a coordinate data of the common vertex and position data for designating a position of the coordinate data of the common vertex; and
a coordinate-transformer for coordinate-transforming the coordinate data of the common vertex and coordinate data disignated by the position data corresponding to the common vertex.
In addition, to achieve the above objective, according to the present invention, it is provided a second image display apparatus for displaying an object constituted by a plurality of polygons comprising:
a memory for storing data of vertex in common to a plurarity of polygons as a coordinate data of the common vertex and position data for designating a position of the coordinate data of the common vertex; and
a coordinate-transformer for coordinate-transforming the coordinate data of the common vertex and using the coordinate-transformed instead of coordinate-transforming coordinate data disignated by the position data corresponding to the common vertex.
It is preferable that the coordinate data of the common vertex is inputted to the coordinate-transformer unit earlier than the position data is inputted.
Further, according to the present invention, provided is a memory medium storing data for displaying an object constituted by a plurality of polygons, vertex data of a vertex in common to a plurarity of polygons included in the data comprising;
coordinate data of the common vertex; and
position data for designating a position of the coordinate data of the common vertex.